The present invention relates to methods and apparatus for bending pipe and more particularly concerns an improved pressure die therefor.
In a commonly used type of apparatus for bending pipe, such as automobile exhaust pipes, for example, a rear section of the pipe is grasped by a chuck which rotates the pipe and moves it longitudinally for positioning relative to a bending head. The bending head comprises a rotatably mounted bend die and an opposed clamp die which clamp a portion of a tube to be bent therebetween, so that upon joint rotation of the clamp and bend dies, the pipe is bent about the bend die. A portion of the pipe immediately to the rear of the bend and clamp dies (toward the chuck) is restrained against lateral motion by a pressure or wiper die.
Where the described apparatus is used for draw bending, a mechanism is provided to longitudinally restrain a portion of the pipe immediately to the rear of the portion being bent, so that during the bend the pipe is stretched, or drawn (e.g., tensioned beyond its yield point). Such restraint may be provided by an internal mandrel, the pressure die, or a combination of the two. To this end the pressure die will press the pipe against the bend die with sufficient force so that friction between the pipe and dies tends to restrain, either completely or partially, the forward motion of the pipe as it is bent and pulled around the rotating bend die. Commonly the pressure die is restrained against forward motion and the pressure exerted thereby against the pipe is controlled so that a wiping action occurs, that is, the pipe is drawn through the mating cavities of the pressure and bend dies against the frictional restraint imposed by the force exerted by the collective action of the pressure and bend dies.
In some draw bending operations, wiping action, where the pipe slides through and between tightly pressing bend and pressure dies, results in unacceptable marring of the pipe and in unacceptable wear upon the dies themselves. To avoid these undesired effects, the pressure die may be caused to move forwardly, together with the pipe, as the latter is bent around the bend die. In such an arrangement little or no motion of the pipe relative to the pressure die will occur, thus eliminating or greatly minimizing the wiping action and its adverse effects. But the forward motion of the pressure die, together with the pipe, still may be so controlled as to cause the pressure die, in conjunction with the opposed bend die, to exert a sufficient restraint upon motion of the pipe as to provide the desired drawing action.
When the pressure die is fixed (does not move with the pipe) to provide a wiping action in rotary draw bending, a relatively short pressure die can be employed and therefore, the pipe grasping chuck may be moved relatively closer to the point of bend. This enables the apparatus to bend the pipe close to the point thereof that is grasped by the chuck. On the other hand, when the pressure die is moved forwardly together with the pipe, a short pressure die can be used only for a small degree of bend. This is so because large bends, such as bends of 180.degree., for example, require a large amount of forward motion of the pipe and concommitant forward motion of the pressure die. Such large amount of forward motion will cause a short pressure die to move forwardly beyond the bend die so that frictional restraining force can no longer be exerted by the co-acting pressure and bend dies. Thus a moving pressure die must be relatively long and is accordingly more likely to tend to interfere with the chuck upon making a final bend in a given pipe.
The U.S. Pat. to Suding, et al., No. 3,553,990 suggests a programmed sequence of operations to avoid such interference between chuck and pressure die. In this patent the apparatus is controlled to sense when such interference is likely to occur, whereupon the apparatus is caused to follow a rather complex sequence of operations which include stopping the carriage, completely withdrawing the entire pressure die, advancing the carriage, clamping the tube to the bend die, releasing the pipe from the chuck, retracting the carriage, moving the pressure die to the pipe, making the bend, advancing the carriage to realign the end of the pipe with the chuck, withdrawing the pressure die, advancing the carriage and chuck, and then again grasping the pipe.
The arrangement of the Suding et al patent requires a complex and time-consuming set of procedures which significantly increase the cost and time of bending. Further, this complex set of relative movements of the several parts, grasping and regrasping the pipe and the chuck, withdrawing the chuck from the pipe and completely withdrawing the pressure die from the pipe, may give rise to increased inaccuracies in positioning and re-positioning of the pipe relative to the various machine components and thus create bending inaccuracy. For example, if another bend is to be made after one bend has been made with the chuck withdrawn from the pipe in the Suding et al apparatus, the bend and clamp dies, together with the pipe, must be rotationally backed up to align the free end of the pipe with the chuck. If this alignment is not precise, the advance of the chuck to the pipe for regrasping the pipe may not be possible and may damage the pipe or the chuck.
Accordingly, it is an object of the present invention to avoid or minimize pressure die interference while decreasing or eliminating above-mentioned problems.